Mac Plus FDHD and Other Oddities

[Mac128]

The Guide to the Macintosh Family Hardware indicates that the Mac Plus ROM sockets can handle 1 Mbit ROM ICs and a configuration of two 1 Mbit ROM ICs provides 256KB of ROM.

This is identical to the SE & SE FDHD ROM configuration. Further, the IWM chip used in both the original Macintosh 128K all the way up to the original SE is similar. This same IWM chip is socketed in the SE and can be upgraded to an SWIM chip making it compatible with the 1.4MB FDHD Superdrive.

With that background and perhaps I am rehashing old ground here, but has anyone tried putting SE ROMs in a Mac Plus? More importantly, has anyone tried putting the SWIM ROMs in a MacPlus, along with replacing the IWM chip with an SE SWIM chip? Of course the latter would require de-soldering the old and soldering on a socket. Is there some reason why this would not work? Or is the difference between the Plus IWM and the SE IWM more significant than they appear?

As long as I'm investigating interchangeable parts on a Mac Plus, has anyone tried putting 64K ROMs in the Mac Plus?

Also, the Mac Plus (and the SE for that matter) can be reduced to 512K RAM, basically turning it into a 512Ke with SCSI, by using two 256K SIMMs in one row only. Are there such things as 128K SIMMs to explore a 256K Macintosh, as I've heard was a consideration at one point in the original Macintosh development? I guess 64K SIMMs to emulate a 128K as well would be pushing it, eh?

 

[tomlee59]

As long as I'm investigating interchangeable parts on a Mac Plus, has anyone tried putting 64K ROMs in the Mac Plus?

Also, the Mac Plus (and the SE for that matter) can be reduced to 512K RAM, basically turning it into a 512Ke with SCSI, by using two 256K SIMMs in one row only. Are there such things as 128K SIMMs to explore a 256K Macintosh, as I've heard was a consideration at one point in the original Macintosh development? I guess 64K SIMMs to emulate a 128K as well would be pushing it, eh?

There aren't any such things as 128K SIMMS. Because of the multiplexed address bus of DRAMs, adding one more address pin actually quadruples the address space. So, DRAM chip capacities grow by 4x. 64kbit chips were followed by 256kbit, etc. Making a 128K simm would be possible, but it would be an unnatural act requiring additional logic.

And I have put 64K roms in a Plus board, by accident. Didn't do much testing that way, as I changed things back as soon as I figured out my error. No smoke, though. The Plus powered on fine, but I don't remember much else.

 

[Mac128]

Further reading in the Mac Family Hardware indicates the SE used the C16M clock signal, whereas the Plus was half that speed. This may be the reason people just didn't buy the SE FDHD kit and upgrade their Pluses. Still I wonder if the Superdrive would work at all with a SWIM chip running at half the clock rate on a Plus, or just slowly?

Tom, thanks for the clarification. I thought it was something like that ... so the fabled 256K Mac would have required special decoding to work with the 64K ROMs had Apple decided to go that route with the original Macintosh?

At least I know I can start playing around freely with the ROMs!

 

[tomlee59]

Yes, the SWIM chip uses a 16MHz clock which, as you note, isn't sent to the IWM socket on the Plus. I suspect that feeding a SWIM a half-rate clock would make it not work at all, but that's only a guess. But shipping a 16MHz clock signal over to the chip is not hard (a bit ugly, perhaps), so that isn't a showstopper if you're determined to give it a go. Several accelerators (like the Brainstorm) rely on tapping into the 16MHz clock available on the motherboard, so it can be done.

As to a fabled 256K Mac, perhaps it would have used four banks of 64k bytes each.